There are known window periscope systems which provide a viewer with a line of sight otherwise unavailable to him because of a protected location in which he operates. For example, in the military environment, tanks and armored personnel carriers are equipped with these systems to give the viewer as wide a field of view as possible over the landscape from within his seating compartment. Two types of window periscope systems widely used for these purposes are a movable head mirror type and a tilting body type.
The movable head mirror type of window periscope system is built with an internal movable mirror which is mounted within a fixed body housing behind a front window and relays a view of the landscape to a fixed lower mirror which the viewer observes through a window eyepiece. The viewer's eye level is therefore also fixed, and the size of the field of view depends upon the size of the head mirror. The optics are typically such that a 2:1 ratio exists relating the change in the elevation angle of the line of sight to a given angular movement of the head mirror. Approximately a 65 degree change in the elevation angle of the line of sight is achievable in these systems.
The elevation angle of the field of view in movable head mirror systems is related to the length of the head mirror by the cosine of the pitch angle of the latter. Practically, the elevation angle of the field of view is limited by the construction problems associated with providing a long enough moving head mirror. These systems are characterized by a low moment of inertia of the head mirror in making line of sight adjustments.
The other major type of window periscope is the tilting body type which has fixed head and lower mirrors mounted in a body housing which pivots on bearings around a horizontal axis. The optics provide a 1:1 ratio in the change in the elevation angle of the line of sight for a change in the angle of body tilt, but the viewer's eye must follow the lower mirror through the eyepiece as it moves with the tilting body. This places a limitation on the range of line of sight adjustments based on the limited angular motion capability of the viewer's neck, about 30-35 degrees. Also, the field of view is fixed for any degree of body tilt and the head mirror and front window are relatively small. These systems are characterized by a relatively high moment of inertia for line of sight adjustments.